A rare 29.6-carat blue diamond has been discovered in a South African mine, and it could "be in a class of its own," the Petra Diamonds company said Tuesday. The "exceptional" vivid blue stone was found in the Cullinan mine, near Pretoria, last week and could "yield a polished stone of great value and importance," the company said in a statement. "Whenever you have an item as beautiful, rare and …